In recent years, biometric authentication using a face image, a fingerprint, an iris, a blood vessel pattern, or other part of a person's body as a key has attracted attention as a security system that is convenient as it does not require carrying a key or the like, and is also less vulnerable to improper use due to loss, theft, or the like. In particular, authentication methods using a face image have an advantage that they require almost no time and effort of a person to be authenticated to register biological information required for individual authentication. That is, authentication methods using a face image does not require a finger or a hand to stick out as in fingerprint authentication or a blood vessel pattern, or does not require an eyeball to be directly irradiated with light as in iris authentication. Authentication can therefore be easily carried out even if a person to be authenticated is uncooperative in registration of biological information. Authentication methods using a face image also have an advantage that those who are authenticated have less psychological resistance compared to authentication using a fingerprint, an iris, and a blood vessel pattern.
Japanese Patent Laid-Open Application No. 2005-316888 proposes a technique about a face authentication device that authenticates by mosaicing a face image and causing a neural network to learn it. A neural network here is a mathematical model that aims to represent some properties found in the brain function by means of simulation on a computer. This technique described in the prior art document can absorb changes in the position/angle of a face and changes in lighting to perform personal authentication.
Japanese Patent Laid-Open Application No. 2002-183734 proposes a technique about an authentication device that authenticates by changing a threshold for personal authentication according to the frequency of use by a registrant. This technique described in the prior art document can dynamically change the threshold to be used for authentication according to use conditions, and can improve convenience and security.
However, the technique described in Japanese Patent Laid-Open Application No. 2005-316888 may not be able to sufficiently learn depending on the extent of changes in the position/angle of a face and the extent of changes in lighting, so that it may not be able to adequately respond to various environmental changes in an actual environment. The technique described in Japanese Patent Laid-Open Application No. 2002-183734 does not consider measures against changes in the position/angle of a face and changes in lighting at all, as it changes the threshold according only to the frequency of use by a registrant.
For example, in a case where a face image acquired by a surveillance camera is authenticated by a face authentication device, the imaging environment around the surveillance camera changes with time. So, the imaging environment at the time of registration of a face image of a particular person who is registered in advance and the imaging environment at the time of authentication of the same particular person are usually different from each other. In this case, an input face image and a registration face image of totally different persons may be judged to be similar to each other only because the imaging environment for the registration face image is close to the imaging environment for the input face image to be authenticated. That is, for example, suppose that a registration face image registered in the face authentication device was imaged in blue neon light. On the other hand, suppose that an input face image at the time of authentication was imaged in the same blue neon light. In this case, though the input face image is of a person different from the person of the registration face image, the face authentication device might judge that both images are similar to each other.